Heat pipes or heat pipe-type devices operate on closed evaporating-condensing cycles for transporting heat from a locale of heat generation to a locale of heat rejection, using a capillary structure or wick for return of the condensate. Such devices generally consist of a closed container which may be of any shape or geometry. Early forms of these devices had the shape of a pipe or tube closed on both ends, and the term "heat pipe" was derived from such devices. The term "heat pipe," as used herein however, refers to a device of any type of geometry designed to function as described above.
In such a heat pipe device, air or other noncondensable gases are usually removed from the internal cavity of the container. All interior surfaces are lined with a capillary structure, such as a wick. The wick is soaked with a fluid which will be in the liquid phase at the normal working temperature of the device. The free space of the cavity then contains the vapor of the fluid at a pressure corresponding to the saturation pressure of the working fluid at the temperature of the device. If at any location, heat is added to the container, the resulting temperature rise will increase the vapor pressure of the working fluid, and evaporation of liquid will take place. The vapor that is formed, being at a higher pressure, will flow towards the colder regions of the container cavity and will condense on the cooler surfaces inside the container wall. Capillary effects will return the liquid condensate to areas of heat addition. Because that heat of evaporation is absorbed by the phase change from liquid to vapor and released when condensation of the vapor takes place, large amounts of heat can be transported with very small temperature gradients from areas of heat addition to areas of heat removal.
Many heat pipe applications require both a high capacity and variable conductance characteristics obtained through the use of noncondensable gas. Generally, high capacities are attained through the use of arterial wick structures. The presence of gas, however, aggravates what are already difficult problems in priming and maintaining a primed state of the arteries, particularly with a high pressure fluid such as ammonia.
Because cavitation is not a problem with low pressure fluids, reliable gas-controlled arterial-wick heat pipes can be made using methanol as the working fluid. These heat pipes exhibit axial heat transport capacities on the order of 5,000-7,000 watt-inches, limited by the relatively poor thermodynamic properties of methanol in combination with constraints associated with the priming mechanism.
To achieve higher capacities, as required in many applications, it is necessary to utilize ammonia as the working fluid. In the case of ammonia, however, its high pressure at relevant temperatures promotes pressure fluctuations in heat pipes sufficient to cause cavitation in the arteries and consequent depriming.